Electrically powered furnaces are used extensively in various processes for producing integrated circuits on semiconductor wafers. In some cases, these furnaces are used to cure wafers following a process step, while in other cases the furnaces are used to heat the wafers to a particular temperature during a process, such as oxidation or re-flow conducted on the wafer surface. Many of such processes require relatively close temperature control, otherwise the results are adversely effected. For example, when oxides are being deposited on a wafer, if there is a dramatic shift in temperature during processing, the thickness of the deposited oxide may be adversely affected, e.g. the deposited thickness is below the specified requirements.
Unexpected temperature variations in furnaces used to conduct wafer processing may result from a variety of causes. In many cases, one or more electric heating elements may fail or may operate intermittently, which in turn reduces furnace temperature. These failures are sometimes due to deterioration of heating coils due to age, which results in the coil experiencing an open circuit at the location where the coil has failed. Such temperature variations can be even more significant in so-called vertical furnaces in which the wafers are passed through a plurality of separate, vertically oriented heating zones in the furnace.
In the past, short-term swings in furnace temperature affecting wafer processing were recorded on data logs that were reviewed by an operator at the end of a process cycle. The recorded temperature swings were analyzed and appropriate corrective actions were taken by the operator prior to processing the next batch of wafers. The corrective action could include altering processing parameters for the next batch, or, where the temperatures swing was due to faulty equipment, making appropriate repairs to the furnace. Unfortunately, since the data log was reviewed only after processing cycles were completed, the entire batch of wafers often had to be scrapped since corrective action could not be taken until the process was completed.
Accordingly, there is a clear need in the art for a system that monitors the furnace heating coils on a real-time basis, and notifies an operator when an open circuit has been detected. The present invention is directed toward satisfying this need in the art.